This invention relates to FM broadcasting systems and, more particularly, to an improved FM stereophonic broadcasting system which increases the broadcast coverage area over that of current biphonic service yet is compatible with existing monophonic and biphonic receivers in the presence of multipath propagation.
U.S. Pat. No. 4,485,483, the disclosure of which is hereby incorporated herein by reference, describes a stereophonic broadcasting system incorporating companding of the difference signal which is compatible with existing receivers and which through signal-to-noise improvement significantly extends the area of coverage of an FM broadcasting station. In the patented system, and as illustrated in FIG. 1, stereophonically-related audio frequency source signals L and R are matrixed to obtain stereophonic sum and difference signals M and S, respectively. At the transmitter, the difference signal is used to amplitude-modulate a first sub-carrier signal and at the same time is applied to a compressor which compresses its dynamic range to produce a compressed difference signal S'. The compressed signal S' is used to amplitude-modulate a second subcarrier signal of the same frequency but in quadrature phase relationship with the first. Suppressed-carrier, double-side-band modulation is employed, with the frequency of the sub-carrier signal being sufficiently high to assure a frequency gap between the lower sidebands of the modulated sub-carrier signals and the M signal. A conventional low-level phase reference pilot signal, lying within the aforementioned frequency gap is provided for detection purposes at the receiver. The M signal, the two modulated sub-carrier signals, and the pilot signal are frequency modulated onto a high frequency carrier for the purpose of transmitting the same to one or more remote receivers. The receiver includes a demodulator for deriving the M signal, the normal difference signal S and the compressed difference signal S', and an expander for complementarily expanding the derived compressed difference signal. The expanded noise-reduced version of the difference signal is combined with the received sum signal M to obtain the original audio frequency source signals L and R. In addition to improving the quality of the received signal, the system increases the broadcast coverage area over that of current biphonic service.
Commonly assigned U.S. Pat. No. 4,602,380, the disclosure of which is hereby incorporated herein by reference, describes compressors and expanders useful in the above-described system and teaches the concept of combining, at the receiver, the usual stereo difference signal S and the compressed stereo difference signal S' and then expanding the resulting signal to obtain a noise-reduced difference signal for matrixing with the sum signal. Commonly assigned U.S. Pat. No. 4,602,380, the disclosure of which is also hereby incorporated herein by reference, describes the use of the difference signal S as a reference signal for controlling the expansion of the received compressed difference signal S' so as to cause the amplitude of the expanded difference signal to equal the level of the uncompressed difference signal, making the expander adaptive to any compression characteristic that might be employed at the transmitter.
The system described in these three patents is compatible with conventional receivers provided they are properly aligned so as not to detect the added compressed difference signal; however, a problem can arise when the alignment of the receiver fails to maintain the proper phase relationship between the pilot signal and the sub-carriers. Under such circumstances, there may be crosstalk of the compressed signal into the uncompressed difference signal and, depending on the direction of the phase misalignment, may add to or subtract from the usual difference signal. The magnitude of the crosstalk will be affected both by the amount of compression (gain) of the compressed signal and the magnitude of the alignment phase error. If the phase error is negative, the crosstalk will be out-of-phase and may contribute to a potential narrowing of the apparent stereo stage width. If the phase error is positive, the crosstalk will be in-phase and may contribute to an apparent widening of the stereo image. In actual practice, neither effect is likely to be noticed because the alignment of most receivers falls within a tolerable range.
The problem is more severe, however, with reception in moving vehicles because of the multipath propagation phenomenon, a condition in which a receiving antenna is sensitive to both a direct transmitted signal as well as to multiple, delayed reflections of that signal caused by terrain factors or manmade structures. Depending on delay intervals, multipath propagation can decrease the level of the received RF signal so as to cause noisy reception or complete signal dropouts. In conventional stereo receivers, the effect is characterized by momentary bursts of noise as the vehicle moves through the multipath space. In addition to this RF signal fading, the summation of the multipath signals at the receiver may also distort the phase relationship between the pilot signal and the stereo difference signal. If the transmitted signal also includes the added compressed difference signal S', such momentary phase errors can result in momentary bursts of crosstalk as well as noise. Since the level of the compressed difference signal is generally higher than that of the uncompressed signal, if the phase error is such as to cause crosstalk summation of the two stereo difference signals, loud bursts of sound may be heard. The effect can be lessened if the overall compression (gain) of the difference signal in the quadrature channel is reduced, but to do so would also reduce the effectiveness of the noise-reduction function.
It has been observed that, in general, the most common and deleterious multipath is characterized by relatively short delay times and a low ratio of desired-to-undesired (D/U) signals. Such reception is usually encountered in urban environs where steel frame buildings nearby act as efficient reflectors of the RF signals. It has also been observed that the crosstalk is audibly more objectionable when the audio signals in the compressed and uncompressed channels are in phase with each other, and that the effect is lessened when these signals are 180.degree. out-of-phase with respect to each other. This observation may be explained in terms of the above-mentioned phase error of the pilot tone.
Employing the analysis technique described in an article by T. Bossert entitled, "Impairments to VHF/FM reception in motor vehicles caused by multipath propagation and possibilities for improving receivers", EBU Review-Technical, No. 205, June 1984, and referring to FIG. 2, the curve illustrates the effect on a 19 kHz pilot tone of a second attenuated and delayed 19 kHz signal. The desired or direct signal is represented by vector D, and the undesired signal, represented by vector U, is delayed by the angle .alpha.. The vectorial sum of these two signals is represented by the dashed line vector labeled .SIGMA.. For purposes of this illustration, the undesired signal U is shown to be at a level 3dB lower than that of the direct signal and delayed in time approximately equivalent to the propagation time of an added mile of signal path. It can be seen that the resultant phase error .phi. will always be positive until the delay times exceed one-half the period of the pilot tone, or approximately 26 microseconds, which is equivalent to a D-U path difference of nearly five miles. It is also to be expected that as delay times approach this larger value, the amplitude of the undesired signal will decrease due to the attenuation factor of the longer propagation path. Consequently, the error angle .phi. is not likely to change sign for most multipath signals. Given this analysis, it is therefore desirable to configure the audio phase in the compressed difference channel so as to minimize degradation in compatible reception.
As stated earlier, the degradation of the crosstalk effect on the conventional difference signal can be minimized by reducing the gain of the compressed signal, but if the gain at all frequencies were reduced by a like amount the effectiveness of the noise reduction would be lessened. Applicant has recognized, however, based on his earlier analysis of the frequency sensitivity of the human hearing response, that it is possible to reduce the level of low frequency information in the compressed difference signal without affecting the perceived noise reduction. This is possible because of the frequency sensitivity of the human hearing mechanism, exemplified by the equal loudness contours derived by applicant and others illustrated in U.S. Pat. No. 3,594,506 entitled "Loudness Level Indicator," the disclosure of which is hereby incorporated by reference. Equal loudness contours graphically depict the measurement of levels of sound of equal loudness as a function of frequency and intensity and may be obtained by subjecting test teams to octave bands of pink noise, pink noise being characterized as having equal energy distribution per octave band. Such equal loudness contours indicate that the human ear is less sensitive to a given loudness level at sound frequencies below about 1,000 Hz than it is to sound frequencies about 1,000 Hz; applicant has recognized that this effect can be utilized to reduce crosstalk from the compressed difference signal S' to the difference signal S, in conventional FM stereo receivers, without lessening the perceived noise-reduction at the receiver of the extended range system.
A primary object of the present invention is to provide an improved FM stereophonic broadcasting system incorporating companding of the difference signal which exhibits improved compatibility with conventional FM stereo receivers in the presence of multipath propagation.